This application relates to a power floor or wall, sanding, buffing, leveling or polishing apparatus, and, particularly, to a back-up assembly unit which carries a sandpaper disc in a power sander. The term “sander” (or sanding) encompasses grinding, planing, buffing, leveling, polishing, and the like.
Conventionally known power sanders include a drive mechanism and a back-up pad with an abrasive disc secured thereto. Coated abrasive back-up pads known in the art include a support surface to which sheets having coated abrasive material on one surface and pressure sensitive adhesive on the other may be releasably adhered. Typically the support surface is formed on a flexible polymeric adhesion layer attached to one surface of a layer of resiliently compressible foam. The back-up pad has a rigid annular backing plate attached to an opposite surface of the layer of foam. The back-up pad is then attached to the drive mechanism by a circular array of screws so that the drive mechanism can be used to drive the pad with the abrasive against a surface to be abraded, while the layer of foam provides a flexible cushion causing the abrasive to follow and level the contour of that surface. Such back-up pads are commercially available from National Detroit, Detroit, Mich., and typically include a backing plate made of aluminum, which can bend if the pad is dropped or otherwise impacted edgewise against a solid surface. In another commercially available back-up pad (available from Eezer, Fresno, Calif.), the backing plate is made of a fiber reinforced polymeric material (i.e., fiberglass reinforced epoxy), which can withstand such impacts.
The above-described commercially available back-up pads having solid rigid backing plates tend to level out both large and small curvatures of the surface to be sanded. However, while it may be advantageous to remove smaller curvatures to create a smoother resulting surface, trying to smooth out larger curvatures and slopes results in an unnecessary waste of material, especially when used on wood surfaces. Further, sanding of large curvatures with a sanding device having a rigid plate may results in a larger (and, possibly, unsafe) counterforce acting against the user of the device. Alternatively, when the abrasive disc is secured to the foam skin layer (i.e., the layer which is very flexible), some sections of the foam may be less flexible than adjacent portions of the foam. This results in areas of greater pressure alternating with areas of lesser pressure. Accordingly, when the such flexible pad is used to press moving abrasive material against a surface it results in uneven grinding of the surface.
One conventional example of a back-up assembly is disclosed in U.S. Pat. No. 3,808,753, which discloses a rotary abrasive tool with a flexible coated adhesive disc and a backing assembly. The flexible coated abrasive disc is mounted in the assembly outwardly of a slotted flexible backing disc or pad member which is provided with radially arranged circumferentially spaced slots which extend only partially through the body of the slotted flexible backing disc in the outer peripheral or marginal portion thereof. The slotted, flexible backing disc or pad member is adhesively attached to a second solid or unslotted backing disc or pad member. A second slotted backing disc or pad member is attached to the solid unslotted backing disc at the rear of the solid disc. Radially arranged slots in the two flexible slotted backing discs or pad members are staggered circumferentially relative to each other. Because the unslotted backing disc is formed solid, it suffers from the same drawbacks as described above with respect to back-up assemblies having a rigid backing plate.
Accordingly, there is a need in the art to provide a back-up assembly providing enough rigidity to accomplish smooth and even sanding of curved and uneven surfaces, while allowing for a safe operation and having adequate flexibility during sanding of relatively large curvatures.